sis900: add support for ethtool's EEPROM dump

Implement ethtool's EEPROM dump command (ethtool -e|--eeprom-dump).

Thx to Andrew Lunn for comments.

Signed-off-by: Sergej Benilov <sergej.benilov@googlemail.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>